Session 17 - The Galactic Interstellar Gas.
Display session, Monday, January 13
Metropolitan Ballroom,

## [17.12] HST/GHRS Observations of 61 Cyg A and 40 Eri A

B. E. Wood, J. L. Linsky (JILA, U. Colorado)

We present new HST/GHRS observations of interstellar absorption lines seen in UV spectra of 61 Cyg A (K5 V) and 40 Eri A (K1 V). These include the Lyman-\alpha lines of H I and D I, and the Mg II h and k lines. We use these data to measure the properties of the local interstellar medium (LISM) and to search for absorption from hydrogen walls'' surrounding the stars created by the interactions between the stellar winds and the LISM. The two target stars were chosen on the basis of their close proximity (d=3.5 pc and d=4.8 pc) and their large radial velocities (V_rad=-64 km s^-1 and V_rad=-42 km s^-1).

For each star, the large negative radial velocity creates a large velocity separation between the stellar Lyman-\alpha emission line and the LISM absorption, which means that the wings of the interstellar H I absorption feature preferentially absorb the red wing of the line, thereby creating an apparent blueshift of the wings of the Lyman-\alpha emission line with respect to the star. We expect the intrinsic stellar emission to be centered on the radial velocity of the star. By trial and error, we can determine the H I column density necessary to recenter the emission line wings on the star and thereby measure the H I column density without estimating the full stellar Lyman-\alpha line profile, which is generally the largest source of systematic error in measurements of the H I column density toward cool stars.

High radial velocity stars are also excellent targets for studies of stellar hydrogen walls. The large velocity through the LISM creates a very hot hydrogen wall and a large velocity separation between the LISM and hydrogen wall absorption components, thereby producing a broad, easily detectable absorption feature.

This work is supported by grant GO-06617.01.95A to the University of Colorado.